Demystifying the Debate with PZ Myers Over Evolution and Embryology

This series began when ENV writer Jonathan M. asked some tough questions of PZ Myers about evolution and embryology at a Skeptics event in Glasgow, Scotland. As a recap of this series, below are links to ENV articles which have appeared on this topic:

Previously, I discussed how PZ Myers commonly attempts to shout down his opposition through name-calling and incendiary rhetoric. PZ's use of this methodology has become so extreme that he's shocked authors in mainstream scientific journals and left-leaning pro-evolution media outlets like the NY Times. Recently, PZ tried to use this method when attacking pro-intelligent design undergraduate student Jonathan M. at a talk before a skeptics group in Glasgow, Scotland.

Why does PZ resort to character assassination rather than simply allowing civil dialogue? I think the answer lies in the fact that the empirical evidence is not as clearly on PZ's side as he'd like it to be. He deflects from evidential weaknesses by turning up the volume via extensive name-calling and personal attacks.

And in the case of PZ's recent debate with Jonathan M., we're not just talking about any evidence. We're talking about evidence that is near and dear to PZ's heart because it pertains to the very existence of concept after which he chose to name his blog: the pharyngula.

Another tactic PZ commonly uses when debating embryology is to distort the arguments of intelligent design (ID) proponents by constantly claiming that they are simply attacking Haeckelian recapitulation theory. Recapitulation theory is the idea that as organisms develop, they replay their evolutionary history. Thus we have Haeckel's famous dictum, ontogeny [i.e. development] recapitulates [i.e. replays] phylogeny [i.e. evolutionary history] - a concept which has long been known to be false.

Since modern evolutionary biology has rejected Haeckel's recapitulation theory, PZ claims that ID proponents are only knocking down a straw man argument. The problem with PZ's rebuttal is that we're not attacking recapitulation theory, but rather the argument for common ancestry based upon shared similarities in development--an argument which appears in virtually every mainstream biology textbook, and one which PZ is known to make in various forms. (I realize PZ denies making this argument, but as we'll see throughout this series, he nonetheless makes it.) Though PZ accuses us of attacking a straw man, by misconstruing our arguments and wrongly claiming that our they only entail an attack on Haeckel's recapitulation theory, it is PZ who is promoting the straw man.

To summarize this debate, let's briefly review some of the main arguments on both sides:

Our Point (1): We are NOT arguing that modern evolution biology or embryology is based upon Haeckel's recapitulation theory. Although we do argue that textbooks should not use Haeckel's inaccurate drawings, which overstate the degree of similarity between vertebrate embryos, we recognize that the case for common ancestry no longer depends upon recapitulation theory. As Jonathan M. explains, "For those who want the bottom line, here it is. Myers thinks I'm worried about Haeckelian recapitulation. But that's completely wrong."

Our Point (2): Leading lights of evolutionary biology (in particular, mainstream biology textbooks) commonly claim that vertebrate embryos have a high degree of similarity in their early stages, and that this demonstrates their common ancestry. (If you doubt what I'm saying, all this will be amply documented in this series of articles.) We reply by observing that these claims are inaccurate since vertebrate embryos show many differences, including significant differences from their earliest stages of development to the later stages. Sometimes we point out that evolutionary biology has difficulty explaining supposedly homologous structures which are produced by non-homologous developmental pathways or supposedly homologous developmental pathways that produce widely divergent (even non-homologous) structures.

Our Point (3): Many textbooks and evolutionary authorities claim that one particular stage during vertebrate development is highly similar (or "conserved")--the pharyngular stage--and this provides good evidence that vertebrates do in fact share a common ancestor. We respond by noting that over the past decade or so, some extremely prominent evolutionary developmental biologists have published scientific papers finding that vertebrate development is so divergent that it's doubtful that a conserved "pharyngular stage" even exists.

It's worth reiterating the last sentence in point 3: Not only do vertebrate embryos show many differences early in development, but some leading embryologists argue that vertebrate embryos develop so differently that the pharyngular (also called "phylotypic" or "tailbud") stage may not exist.

For PZ, Point 3 is very problematic. Dr. Myers named his blog "Pharyngula," suggesting this concept is of great persuasive importance to him when it comes to the evidence for evolution. Thus, it comes as little surprise that whenever one debates embryology with PZ, he quickly descends into extensive public name-calling, incendiary rhetoric, and distortion of our arguments in order to avoid debating that last point. PZ's responses to our 3 points thus usually go something like the following:

PZ's Response to Point (1): PZ typically denies that we are making Point (1), and instead argues we are arguing precisely the opposite of Point 1, setting up Haeckelian recapitulation theory as the bedrock of modern evolutionary biology. On this point, he simply ignores our actual arguments. Extensive name-calling usually follows. As PZ wrote while misconstruing Jonathan M's arguments, "Evolution does not predict that development will conserve the evolutionary history of an organism, therefore your question is stupid." Even after Jonathan M. made it clear that this is not what he's arguing, PZ replied by misconstruing it as such, claiming that Jonathan M. is "completely ineducable."

PZ's Response to Point (2): Frankly, usually PZ raises such a ruckus in response to Point 1 that he doesn't often get around to addressing our Point 2. And depending on which day you catch PZ on, you might get a different answer. In the past PZ has responded to our Point 2 by denying that early stages of embryos show wide variation, and instead claiming that the differences between early stages of vertebrate embryos are merely "superficial." But in his recent response to Jonathan M., PZ changed his tune. He admitted that the early stages of development can "vary greatly" and even show "wide variation." PZ's more recent admission about greater variance among early vertebrate embryos also led him to explicitly take a weak stance on whether evolutionary biology predicts we should find similarities when comparing vertebrate embryos. Thus, another success of Jonathan M.'s recent debate with PZ before the Glasgow Skeptics was that Jonathan M. forced PZ to reply squarely to our Point 2. Here was PZ's reply: "I wish I could get that one thought into these guys heads: evolutionary theory predicts differences as well as similarities." This is rich: If PZ is right, and evolutionary biology predicts both similarities and differences between vertebrate embryos, then it would seem that evolutionary biology really predicts nothing at all about development and is unfalsifiable regarding the evidence from vertebrate development. According to PZ, evolutionary theory predicts whatever it predicts, conserves whatever it conserves, and modifies whatever it modifies. Some theory.

PZ's Response to Point (3): In light of PZ's response to our Point 2 (that "evolutionary theory predicts differences as well as similarities" in vertebrate embryos), one might reasonably presume PZ views evolutionary processes as so plastic and unpredictable that one cannot really make an argument for, or (lucky for him) against, common ancestry based upon the similarities (or differences) between vertebrate embryos. But this is wrong. In what appears to be another contradiction, in response to Point 3 PZ has argued that "substantial similarities" between vertebrate embryos during the pharyngula stage "are evidence of common descent." He has asserted these "substantial similarities" are "a fact," and levied his fury towards those who would cite scientific papers questioning the existence of the pharyngular stage. To my knowledge, PZ has yet to address the papers we've cited which challenge the existence of a pharyngular stage. PZ's use of incendiary rhetoric while avoiding our scientific arguments might indicate that our Point 3 hits close to home. Clearly, the evidence for the pharyngula is very important to PZ.

In some subsequent articles, I'll elaborate on the evidence regarding each of these 3 points.